Increasing numbers of portable electronic devices have increased the requirements for various charging schemes for the devices. Most of these devices function in a fashion wherein they may have a first mode of operation wherein the devices are directly connected to an AC/DC power adaptor that is plugged into a wall socket. While connected, the AC/DC adaptor enables the portable electronic device to operate off of the provided AC power and additionally enables charging of a battery within the portable electronic device. Once the battery has been at least partially charged, the electronic device may be powered by the battery. In this way, the device may be unplugged from the AC/DC power adaptor and moved about enabling the user to use the portable electronic device in a number of locations which may or may not have associated power sources.
One method of architecting the power in a portable device is called narrow rail VDC (NVDC). One aspect of NVDC is that the battery is always connected to the system rail voltage unless the battery is fully charged. The system rail is regulated to the battery voltage instead of the adaptor voltage. This architecture has the benefit that the ratio of maximum to minimum system rail voltage is smaller than in a conventional power architecture. This results in higher efficiency of the regulator sitting on the system rail and longer battery life for the portable electronic device.
One challenge with the NVDC architecture is the ability to charge an extremely discharged or shorted battery without collapsing the system rail. The system rail may not fall below seven volts or the converters on the system rail may not work properly. Additionally, the battery may be charged to six volts and need to be trickle charged. Trickle charging involves charging the battery at a much lower rate to bring a lithium ion battery out of a deep discharge state. Prior art configurations (see FIG. 1.) included circuitry that initiates trickle charging when battery voltage falls below a threshold voltage of typically eight volts. When the battery voltage is low, trickle charging is initiated responsive to the drop below the threshold voltage. This causes a corresponding drop in the charging current. Since the charging current has fallen below its quick charge limit, the charger will regulate the system rail to the charging voltage typically 12.6 volt. The battery will be at its lower voltage, and a resistor 134 limits the trickle charge current.
There are three primary weaknesses with this method of charging. First, the charging current is not regulated and changes as the battery charges. The charging current is equal to 12.6V minus the voltage of the battery divided by the resistance through which the charging current is flowing. The resistance is sized so that the charging current is at the battery trickle charge spec, typically 100 milliamps, when the battery voltage is at zero volts. Since the charging current reduces as the battery charges, the charging current will fall well below the specified trickle charge current level by the time the battery has charged to approximately eight volts. In this typical case, the charging current falls to 36% of the specified trickle charge current. This low current may confuse some charging algorithms into thinking that the battery is damaged beyond repair. Additionally, this low current causes the battery charging time to be exceedingly long. A second problem arises because the maximum power dissipated by the resistance when the battery is at zero volts is 12.6 volts times 100 milliamps or 1.26 watts. This requires special thermal considerations. Finally, two PMOS switches are required to isolate the battery from the system rail when the battery is fully charged and an adaptor is present. The additional PMOS transistor switch results in more cost, higher power loss, and shorter battery life for the circuitry. Thus, an improved method for trickle charging a battery within a narrow rail architecture is desired.